Heat a large skillet over medium heat. Add the walnut pieces to the hot dry pan and stir occasionally until toasted and fragrant. About 5 minutes. Remove from the heat and set aside.

Bring a large saucepan of salted water to a rolling boil. Add the green beans and cook for about 6 minutes, or until crisp-tender. Drain and set aside.

In the same skillet you toasted the walnuts in, heat the butter and olive oil until the butter melts. Add the chives and saute for about 30 seconds. Whisk in the maple and apple cider vinegar. Add the green beans and toss until coated. 

Add the toasted walnuts and salt and pepper to taste. Remove from the heat, transfer to a serving platter and serve hot.